Federal law enforcement officials say they are no closer to determining the source of anthrax that killed four people in the United States, than they were when the first case was diagnosed weeks ago. More than a dozen other people are known to be infected. U.S. anti-terrorism officials appeared before a Congressional subcommittee Tuesday and offered a grim assessment of how little they know about where the anthrax may have come from.

A senior Federal Bureau of Investigation official told lawmakers he does not believe the anthrax sent in letters to Senate Majority Leader Tom Daschle and a number of media outlets was stolen or misplaced from a registered U.S. laboratory.

But in testimony to a Senate Judiciary subcommittee and under sharp questioning from the chairwoman, Democratic Senator Dianne Feinstein of California, James Caruso admitted his agency still does not know where the anthrax came from or who is responsible. "We are still searching for that answer," he said. "With close cooperation from the CDC [Centers for Disease Control] and other institutions, we are learning more about the characteristics of anthrax and with that we are in a position to ask smarter questions and get better answers."

"How many labs handle anthrax in the United States?" asked Senator Feinstein.

"We do not know that at this time," responded James Caruso.

"You do not know that?" wondered Dianne Feinstein.

"No, we do not," said Mr. Caruso. "We are pressing hard to determine that."

"The research capabilities of thousands of researchers is something we are continuing to run down," said James Caruso. "I know it is an unsatisfactory answer, and unsatisfactory to us as well."

Mr. Caruso, the FBI's deputy assistant director, says his agency is at a loss to explain the source of the anthrax despite having 7,000 investigators working on the matter.

Mr. Caruso says over the years many people have passed through U.S. laboratories and research facilities, where they could have learned about or had access to anthrax.

He says his agency has received a flood of hoaxes, straining FBI resources and making its task all the more challenging. "Since mid-September, the FBI has responded to approximately 7,089 suspicious anthrax letters, 950 incidents involving other WMD [weapons of mass destruction] matters such as bomb threats, and an estimated 29,331 telephone calls from the public about suspicious packages," he said. "The vast majority of these responses were not actual incidents."

Mr. Caruso, and another official, James Reynolds, who heads the Justice Departments's anti-terrorism division, called on lawmakers to pass tougher laws to crack down on hoaxes.

The U.S. Congress recently passed, and President Bush signed, broad anti-terrorism legislation that, among other things, outlaws possession of a number of biological materials.

Senator Feinstein, arguing that the measure does not go far enough, has introduced a bill to tighten regulation of various germs, fungi and toxins that could be used as weapons.